2.5 CELL CYCLE
Cell Division and Mitosis
WHY DO CELLS DIVIDE?
When organisms grow, their cells don’t get bigger
 Instead, the cells divide (more cells)
 Why?
 So each cell stays small, and manageable
 To replace old or damaged cells
 For reproduction
 For multicellular organisms to grow (all cells require
nutrients and must get rid of waste)

CELL DIVISION FOR GROWTH
Chemicals must pass through
the cell membrane to get in and
out of the cell
 Diffusion
 Chemicals move from an
area of high concentration to
low concentration
 If there is a high
concentration of substances
outside the cell, some
particles will move into the
cell, and vice versa.

CELL DIVISION FOR GROWTH

Osmosis
 Diffusion of water
 Water moves in the
direction of higher solute
concentration
 EX: toward the most
concentrated, away from
the most dilute
THE IMPORTANCE OF CELL DIVISION
Reproduction
Asexual
Sexual
1 Parent
2 Parents
Genetically identical offspring
Genetically different offspring
CHROMOSOMES
 Structures
found in the nucleus
 Made up of DNA (deoxyribonucleic acid) and
protein
 DNA contains the genes that provide the
instructions for the traits that the cell or organism
has
 Before division, a copy of the genetic information is
made so each of the 2 new cells have its own copy
 Humans have 46 chromosomes
CHROMOSOME TERMINOLOGY
23 + 23 = 46
You have one copy
of each chromosome
from each parent!
Eye Colour
Eye Colour
Hair Colour
Hair Colour
CELL CYCLE
Interphase
 G1
 S
 G2
 Mitosis
 Prophase
 Metaphase
 Anaphase
 Telophase
 Cytokinesis
 Division of cytoplasm and organelles
 Results in formation of 2 identical daughter cells

MITOSIS
OVERVIEW
INTERPHASE
Period between cell divisions
 G1 – growth phase (4 hr)
 S – growth and DNA replication (10 hr)
 G2 – growth phase and preparation for division (4 hr)

MITOSIS  PROPHASE

Just before mitosis begins, the
chromatin coil

The long strands form short and
thick chromosomes

They appear as a pair of
identical, copied strands attached
at a single point (centromere)
MITOSIS  PROPHASE (PROMETAPHASE)
Nuclear membrane and nucleolus disappear
 Centrioles separate and travel to opposite poles
(plant cells do not contain centrioles)
 Spindle fibres attach to the centromere

MITOSIS  METAPHASE
Chromosomes line up at equator (middle of cell)
 Mitotic spindle fibres attach to centromeres

MITOSIS  ANAPHASE
Sister chromatids are pulled to opposite poles
 Centromeres divide in half

MITOSIS  TELOPHASE
Sister chromatids are at opposite poles
 DNA becomes thin (chromatin)
 Nuclear membrane reassembles

CYTOKINESIS
Division of cytoplasm
 Animals
 Cell membrane pinches in and
forms a cleavage furrow


Plants

Cell plate forms between the
two daughter cells and will
eventually become a cell wall
RECAP-- MITOSIS UNDER
THE MICROSCOPE
Prophase
Metaphase
Anaphase
Telophase
CELL CYCLE
CHECKPOINTS IN THE CELL CYCLE
cell’s activities are controlled at specific checkpoints
 Specialized proteins monitor cell activities and send messages to
the nucleus
 The nucleus then instructs the cell to divide or not
 Cells remain in INTERPHASE (do not divide) if

1 signals from surrounding cells say “don’t divide”
 2 not enough nutrients
 3 DNA has not been replicated
 4 DNA is damaged
NOTE: If DNA is damaged and it’s early in cell cycle, the cell will repair
itself OR destroy itself (keeps organisms healthy)

WATCH THIS!
 Go
to http://www.cellsalive.com/mitosis.htm
for a mitosis interactive animation